A network is used to supply a communication path between two end users. The network typically has the capacity to connect only a finite amount of communication traffic at any given instant. Each call consumes some portion of the total available communication resources. Moreover, a network may have numerous diverse resources available for use in forming any single call's communication path. Thus, networks often perform a call setup process prior to establishing a call's communication path. The call setup process identifies and assigns network resources to the upcoming call.
A network may be viewed as having a number of nodes. The end parties to a call communicate directly with their respective nodes. In some cases, the calling and called parties utilize the same network node. However, in other cases communication paths are established between diverse nodes, perhaps through intermediary nodes.
Call setup processes are well known in connection with networks having stationary end users. In this situation, a calling party need only identify a called party to one node of the network. That one node can analyze the called party's identity to determine how to begin identifying and assigning network resources to setup the call.
However, the call setup process becomes more difficult when one or more of the end users are mobile. When mobile end users are involved, RF links are typically used to communicate with the end users. These RF links represent scarce resources which must be conserved to the maximum extent possible. Since the end users are mobile, the locations of called parties cannot be determined simply by examining the called parties' identities. Hence, additional network resources must be consumed and additional intelligence must be designed into the network to determine how to setup a call to a called party whose location varies.
The call setup process is further complicated when at least some of the nodes of the network are mobile themselves. This situation occurs when satellites in moving-earth orbits are used as the network nodes with which network subscribers directly communicate. In this situation, the selection of particular network nodes through which a call is to be placed depends upon which satellites are in convenient locations at the instant a call is being setup.
Moreover, simple satellite designs are highly desirable to reduce network costs. In addition, simple satellite designs provide the most cost-effective network reliability because the repair of orbiting satellites is prohibitively expensive. Due to a desire to keep satellite designs simple, the complexity of call setup intelligence is preferably located in nodes of the network which are not mobile.